You can verify that your M780 has a Wacom digitizer by going to system properties and looking at the entries for the digitizer.

Sometimes it's challenging installing the drivers, but if you go with the Wacom Pen Enabled drivers for tablet PCs available on Wacom's site it should work (alternative drivers are the Wacom Bamboo). If you have installed those and you don't see a yellow exclamation point next to your digitizer in device manager, you may have to remove the factory drivers, then install the Wacom drivers.